A mini-Explainer on how a little moisture may have blacked out much of Manhattan.

A transformer explosion knocked out power to a wide swath of Manhattan on Monday night. ConEd says it could take up to a week to restore power to tens of thousands of customers. It’s not yet clear whether the explosion was directly related to the storm. How can a hurricane cause a transformer explosion?

By degrading the insulation. Electrical transformers are composed of a series of coiled electrical wires. The wires are sheathed in paper-based insulation, which prevents electricity from jumping across wires within the coil. Over the course of 20 to 60 years, depending on how hard the apparatus is made to work, the insulation in a well-maintained transformer degrades from 1,200 molecules thick to 200 molecules thick, at which point the coils should be replaced. Water accelerates that degradation process immensely. When the insulation fails, parts of the coil touch, causing a “turn-to-turn fault”—a form of short-circuit that creates a spark inside the transformer. The spark ignites the oil surrounding the coils, and the resulting explosion can be massive, as video from Monday night’s failure demonstrates.

It’s not yet clear whether water penetration caused the explosion in Manhattan, or, if it did, how much water was involved. It doesn’t take much, though. Even small amounts of water can quickly be fatal to an electrical transformer. Transformers are usually protected by air dryers to remove humidity from the air, and engineers regularly replace the insulation surrounding the entire machine. Even water released from inside the transformer—as organic materials in the system heat up and age—can pose a problem. Modern transformers are also equipped with a tank of silica gel that absorbs water from air entering the electrical system.